The invention relates to a peptide compound having an improved binding affinity to somatostatin receptors, comprising a somatostatin analogue as the peptide and a chelating group covalently linked to the N-terminal free amino group of said peptide
Such peptide compounds and their radiolabelled derivatives can be used for therapy of somatostatin—receptor positive tumors. Detectably labeled somatostatin—peptide compounds are also useful for in vivo imaging. See in this respect the patent publications of Albert et al. (U.S. Pat. No. 5,753,627; U.S. Pat. No. 5,776,894), of Krenning et al. (U.S. Pat. No. 6,123,916), of De Jong et al. (WO 00/18440), and of Srinivasan et al. (U.S. Pat. Nos. 5,804,157; 5,830,431). Albert et al., disclose complexed somatostatin peptides for in vivo imaging of somatostatin receptor—positive tumors, which peptides are derived from somatostatin analogues, carrying an optionally substituted phenylalanine residue or a beta- or 2-naphthylalanine residue in its 3-position. Selective internal tumor therapy with radiolabelled peptides has become very important in nuclear medicine in the past years. Especially somatostatin derivatives have been successfully applied in the clinic for tumor diagnosis and therapy, showing that the principle of receptor targeting is working in practice. For more than four years already much experience has been gained in clinical trials with the use of 90Y—labeled DOTA-[Tyr3]-octreotide (DOTA-TOC) for tumor therapy (M. de Jong: Eur. J. Nucl. Med. 26, 1999, 693–698). Yet DOTA-TOC only shows high affinity to the somatostatin receptor subtype 2 (sst 2), whereas the affinity to other somatostatin subtypes, in particular sst 3 and sst 5, which are found also in a variety of tumors, is too low to contribute essentially to tumor targeting. For example, most thyroid tumors express these last-mentioned somatostatin receptor subtypes, but have only low levels of sst 2 (E. B. Forssell-Aronsson et al.: J. Nucl. Med. 41, 2000, 636–642).